Re: isolation level serializable

Chuck <skilover_nospam_at_bluebottle.com> wrote:
> I have a stored procedure that copies a bunch of tables from one schema
> to another using "execute immediate 'insert...' " and code that looks
> something like this...
>
> for i in (select table_name from ....)
> loop
> stmt := 'insert into s1.'||table_name;
> stmt := stmt||' (select * from s2.'||table_name')';
> execute immediate stmt;
> end loop;
>
> Some of the tables have FK relationships between them on both the source
> and destination schemas. The procedure uses a similar loop early on to
> disable all constraints on the destination tables, and another later to
> enable them again after the inserts are done. The constraints stay
> enabled the entire time on the source tables.
>
> The problem is that I am getting FK constraint violations when I try to
> enable the constraints on the destination tables. I thought I could
> eliminate this by using a serializable isolation level for the
> transaction. According to the manual, this will cause all queries to
> look at a snapshot of the source tables from the same point in time, so
> I execute this before the first insert.
>
> commit;
> set transaction isolation level serializable;
>
> Then I do another commit at the end of the last insert. There are no
> commits, rollbacks, DDL, or any other SQL between the inserts, however I
> still get FK violations at the end when I try enable the constraints.
> Why? Am I misunderstanding the serializable isolation level? Is there
> something special about "execute immediate" where it ignores the
> serializable transaction setting?

You seem to have misunderstood the concept of serializable isolation.

This is something that only affects what you see when you look at the table, but has no influence on what happens when you or somebody else modify the table.

Setting isolation level serializable does not keep you from entering data that violate consistency, since the foreign key is disabled.

It also won't keep somebody else from modifying data in the tables concurrently so that the result is inconsistent even when the data YOU enter are consistent, again because the constraint is disabled.

If you need to keep other people from changing data concurrently, you lock the table appropriately. It is not a matter of isolation level.

Yours,
Laurenz Albe Received on Wed Mar 21 2007 - 11:57:24 CDT